cmd/vendor: get golang.org/x/tools@f946a157eef

To bring in the fix for sigchanyzer pass to detect valid usage of
unbuffer channel to builtin make.

Fixes #45043

Change-Id: I60d2ee90f7c111183b33747008903a7df88b76ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/312631


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>